The Black Hole Approach: Don't Get Sucked In!

Whether you fly a piston single or a heavy jet, a long straight-in approach at night over featureless terrain is a well-proven prescription controlled flight into terrain. AVweb's Linda Pendleton examines the optical illusions involved, and offers suggestions for making sure that you don't become a thing that goes bump in the night.

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About the Author ...

Linda D. Pendleton is Manager
of Computer Graphics and Animation for
King Schools.
She is also the author of a book, Flying Jets, and scriptwriter for
several of the training videotapes published by King Schools, including
"Navigation from A to Z," "METAR/TAF Made Easy," and "Handling Emergencies."

Linda is an ATP with Citation 500 and Learjet type ratings, and a CFI with
airplane, instrument and multiengine ratings. In her 10,000+ hours of flight
experience, she's flown US Mail, freight, corporate, charter, commuter, and
served as an FAA-designated examiner for the Citation 500.

One of the major tenets of instrument flying is that you cannot rely upon your body —
or kinesthetic senses — to keep you upright. Repeatedly, your CFII has pounded into your
head the premise that you can only believe what your eyes tell you. Just watch those
gauges and all will be well. And it's true — if what you are looking at IS the
gauges. When the view is out the windscreen, however, you cannot always believe your
visual perceptions.

Great! First you can trust your eyes, then you can't. What's the deal here? Well, it's
all in being human. This marvelous species you're so proud to be a part of has evolved
over millions of years as a land-based animal moving at a normal speed of about three to
four miles per hour. You can also manage occasional short bursts of up to 15 mph. Any
time you go faster than that or put your eyes higher than eye level above ground you're
subject to misperceptions.

Okay. So, you know that you can't always trust our perceptions. What is a black hole
approach and what makes it so dangerous? The term "black hole" refers to the
terrain below the approach to the airport, not the airport itself. Simply put, a black
hole approach is a long, straight-in approach at night to a brightly lit runway over
featureless and unlit terrain. Over the years, the black hole approach has claimed the
lives of many pilots — both novice and experienced. Night flying has always been more
dangerous than daylight flying principally because of the lack of perceptual clues we all
depend on to keep the shiny side up. You're all familiar with the false perceptions you
can fall prey to caused by using a sloping cloud deck for a level horizon and the
unsettling ambiguity caused by mistaking sparse ground lights for stars. You can overcome
these visual traps, however, by simply referring to the flight instruments on the panel.
The black hole approach is different in that a glance at the flight instruments won't
always clue you in to the danger.

Figure 1: Which line is longer? (Mouse-over to
check.)

Figure 2: Are lines parallel? (Mouse-over to check.)

Figure 3: Which gray square is darker? (Mouse-over
to check.)

Optical illusions

Before we talk about black hole approaches, let's explore some of the ways your
perceptions can mislead you. That will give us a basis for a better understanding of the
illusions you experience during a black hole approach.

Your eyes really don't do the seeing — your brain does. Your eyes simply transmit
electrical pulses and your brain does the work of making sense of those spikes of
electricity. It perceives what it "sees" in the setting in which it is viewed.
The surrounding objects and colors — or lack of them — will have a big effect on what sense
your brain makes of the electrical impulses sent to it by the retinas. Look at Figure 1.
Decide which line is longer and then roll your mouse over the figure and see the change.
Now you've probably seen this a million times before and you know the answer, but pay
attention to how fooled you brain is by the surroundings of the two vertical lines. Now
look at Figure 2 and decide which lines, if any, are parallel. Again, you probably know
the answer, but notice again the overwhelming perception that the three long lines are not
parallel. Even when you know the answer, the false perception is overpowering. Figure 3
shows the effect surrounding color or intensity has on perception. Note the relative
brightness of the smaller gray squares in the center of the black and white squares. Now
roll your mouse over the figure and note the "change" in the gray squares. The
small square surrounded by black seems brighter and closer than when it is surrounded by
white, but both gray squares are in the same place and are the same color.

Seeing is not believing! But why would the brain play such tricks on you? It's all a
part of how you make sense of the world around you. The visual surroundings of an object
give you valuable clues about its size and distance from you. Lines can show perspective,
which is an indicator of distance. The brightness of an object is another attribute the
brain takes into account when determining the nearness of an object. We perceive dimmer
objects to be farther away than bright ones.

Night perils

Pilots have recognized since the early days of aviation that flying at night is more
dangerous than flying in the daylight. In fact, flying at night in good weather is closer
to a flight in IMC than it is to VMC. The low level of light means that the rod cells in
the retina of the eye are going to be doing most of the work since they are more sensitive
to very weak light energy. Unfortunately, the rods permit seeing only black, white, and
grays. Since you base much of your perception of size and distance on color variation, you
have a handicap already. Terrain and clouds can be almost impossible to see at night until
it's too late and as was said earlier, ground lights can be mistaken for stars and
horizons.

But what makes the black hole approach so different and so lethal? Well, first,
referring to the attitude indicator, altimeter, and turn coordinator won't immediately
alert you to the problem. Pilots who succumb to the black hole illusion are convinced,
sometimes until it is too late, that they are on the proper glide path and all is going
well. Second, although you may know intellectually that the illusion is taking place, you
will still have an overwhelming urge to believe your false impressions. You can't take any
training to keep from experiencing this illusion. Like hypoxia, it WILL happen to you and
your best defense is knowledge and avoidance.

Figure 4: The visual angle subtended by the runway
during a normal three-degree approach should get larger and larger as you continue the
approach.

Many researchers have studied the black hole illusion. Two Boeing engineers, Dr. Conrad
L. Kraft and Dr. Charles L. Elworth, conducted a study in a specially developed night
visual approach simulator flown by Boeing's senior pilot-instructors and came to some
surprising conclusions. As you are aware, pilots flying a normal three-degree glide path
see a constantly changing view of the runway. While the aiming point on the runway will
remain stationary in the field of view, the visual angle occupied by the runway is
constantly changing. Figure 4 illustrates how this visual angle changes during the
approach. (I exaggerated the angles to make the illustration clearer, but the concept
remains valid.)

When black isn't beautiful

Figure 6: If the pilot keeps the visual angle
subtended by the runway constant, the approach path will be an arc.

What Kraft and Elworth discovered is that pilots conducting an approach over
featureless terrain at night tend to keep the visual angle of the runway constant. Now,
I'm going to ask you to think back to high school geometry. Do you remember the theorem
that says that if two inscribed angles intercept the same arc of a circle, the angles are
congruent? Whoa! That was a mouthful — and worthy only of a high school geometry teacher.
Let's look at another picture. Figure 5 shows a circle with an arc AB. Angles ACB and ADB
are inscribed angles that intercept the same arc, AB, and therefore they are congruent, or
equal. Do you see where I'm going here? It follows that if this theorem is true then you
can turn it around to say that if two angles intercept the same arc of a circle and are
congruent, then those two angles are inscribed on the circle, meaning that their vertices
are on the circumferences of the circle.

Now let's look at Figure 6. This shows (although exaggerated for clarity) what happens
when a pilot flies an approach to a runway and keeps the visual angle of the runway
constant. The approach path will be on the circumference of a large circle centered over
the approach area. This means that the descent to the runway will be too steep at first
and will flatten out as it gets closer to the runway. As a matter of fact, the Boeing
researchers found that the typical descent on a black hole approach, if continued to
touchdown, would result in a landing (impact?) two to three miles short of the runway.
Although the circular path is clear in the illustration, it is imperceptible to the pilot
flying the approach.

Although research has not yet discovered why pilots tend to keep the visual angle of
the runway constant under black hole conditions, they have discovered that the condition
is universal. You WILL be fooled if you try to conduct a long, straight-in approach
over featureless terrain using only out-the-window references. There is no amount of
training or practice that will make this illusion go away. Just like the visual illusions
we looked at earlier, you know what the answer is, but your perceptions lie to you repeatedly.
As you have seen, these false perceptions can be overwhelming. The only defense you have
is awareness and avoidance.

Some conditions make the black hole effect more pronounced. Be alert for the illusion
when you observe these conditions:

An airport that is on the near side of a brightly lit city with few or no terrain
features or lights between you and the airport. The brightness of the city lights will
give the impression that they are closer than they are.

An airport that is on the coast or in very sparsely settled terrain such as deserts and
wilderness areas. This is the classic black hole scenario. Los Angeles International
landing to the east and Salt Lake City landing to the south are classic examples.

A night with extremely clear air and excellent visibility. One of the things we use
to judge distance is the normal hazing that distance provides. When the air is extremely
clear, this lack of hazing makes things appear much closer than they really are.

Coping with the black hole illusion

Since you know what sets you up for the black hole illusion, what can you do to keep
from being sucked in? The most obvious is to avoid long, straight-in approaches. The black
hole illusion disappears within two to three miles of an airport so the most obvious thing
to do is to fly to the airport at a known safe altitude and then descend and fly a normal
traffic pattern.

We said earlier that reference to the flight instruments will not help in a black hole
situation and that is true for a quick reference to the attitude indicator, airspeed
indicator or altimeter. Nothing there will be immediately suspicious. If you study the
VSI, however, you may notice a larger than normal rate of descent, but that may not be
apparent. You need to do a little analysis to see the whole picture. A three-degree
descent — 300 feet per nautical mile — is the normal landing descent. If you see more
than that, you should be suspicious. However, what in the cockpit measures descent angles?
Your airspeed indicator and VSI do. For that three-degree descent, your rate should be five
times your ground speed. If you're doing 120 knots across the ground, your rate of descent
should be about 600 fpm. If you don't know your ground speed, using your indicated
airspeed will be close enough to keep you out of trouble. Of course, to use this formula
for a descent to the runway, you have to know how far you are from the runway. DME, GPS,
or good old-fashioned pilotage should be able to tell you that.

There are many other theories about factors that may contribute to the black hole
illusions. Some are more believable than others, but the thing you MUST believe is that if
the conditions are right, you can be fooled by the black hole illusion and the only way to
keep from getting sucked in is to analyze what you see out the windscreen and be aware
that you, too, can be fooled. Seeing is not believing.

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